This invention relates to a nickel-metal hydride secondary battery using a hydrogen absorbing alloy as the negative electrode material and a nickel oxide as the positive electrode material. More particularly, this invention relates to a hydrogen absorbing electrode for use in nickel-metal hydride secondary batteries that has a large charge-discharge capacity, that experiences only small deterioration in characteristics during prolonged repetition of charge and discharge cycles, and that withstands discharge at large current with a smaller drop in discharge capacity.
As a promising candidate for a high energy density storage battery, a nickel-metal hydride secondary battery has been proposed and intensive efforts are being made to develop commercial products. This battery uses a hydrogen absorbing alloy as the negative electrode material, which absorbs and desorbs hydrogen reversibly, and the absorbed hydrogen is used as an active material. The ideal hydrogen absorbing alloy for use in this nickel-metal hydride secondary battery should satisfy the following requirements:
(1) a large amount of available hydrogen should be absorbed/desorbed; (2) the hydrogen dissociation pressure at equilibrium (or the hydrogen equlibrium pressure) should be in the range of from 10.sup.-3 to a few atmospheres at operating temperatures of a battery (-20.degree. to 60.degree. C/);
(3) it should have high resistance to the corrosive action of concentrated alkaline electrolytes;
(4) the pulverization rate due to repeated charge and discharge should be low;
(5) it should not experience compositional changes due to such factors as the dissolution of certain elements during repeated electrode reaction;
(6) the hydrogen diffusion rate should be high enough to cause a small reaction resistance (overpotential);
(7) it should not experience deterioration in characteristics during prolonged storage in the air atmosphere; and
(8) it should be inexpensive.
Among the conventionally known inexpensive materials that contain rare earth elements is misch metal (which is hereinafter sometimes designated Mm for differentiation from Lm of the present invention in terms of the composition of rare earth elements). Misch metal is a mixture of rare earth elements and usually comprise 25-35 wt % La, 45-55 wt % Ce and 10-15 wt % Nd. Since Ce assumes a large proportion of the rare earth element content, a hydrogen absorbing alloy prepared from Mm has a high hydrogen equilibrium pressure. In order to reduce the hydrogen pressure to less than about one atmosphere in the operating temperature range of batteries, part of Ni must be replaced by Co, Mn, or Al (the respective cases are hereunder referred to simply as Co substitution, Mn substitution, and Al substitution). In hydrogen absorbing alloys of the MmNiCoAl system, the absorption of available hydrogen usually decreases with the increasing amount of Co substitution, and batteries using such alloy have a correspondingly smaller discharge capacity, but exhibit a longer cycle life. Hence, it is advisable to minimize the amount of Co substitution within a range where satisfactory cycle life characteristics are maintained. It has been verified experimentally that the minimum and necessary amount of Co substitution for obtaining good cycle life characteristics is 0.6 to 0.7. Further, it has recently been found that alloys having a lesser Co substitution but an increased Ni content have satisfactory quick discharge characteristics.
Aluminum is effective for improving the cycle life characteristics of a battery. It has recently become clear that Al dissolves into alkali electrolytes. Although the exact mechanism for life extension by Al is not known, it would form compounds such as potassium aluminate and act in a way to retard the oxidation of the alloy. However, excessive Al substitution will act in a way to increase the reaction resistance of the electrode, whereby the overpotential is increased to deteriorate the high rate discharge characteristics and the discharge characteristics at low temperatures. Hence, in order to obtain hydrogen absorbing alloys of the MmNiCoAl system that exhibit good electrode characteristics, the Al substitution must also be reduced to the minimum necessary level This approach has been adopted in the development of the alloy MmNi.sub.3.5 Co.sub.0.7 Al.sub.0.8 which, as will be described hereinafter, provides an initial discharge capacity of 254 mAh/g, as well as excellent cycle life characteristics; however, the high rate discharge characteristics of this alloy are not necessary satisfactory. In short, when using Ce-rich Mm as a starting material for hydrogen absorbing alloys, a considerable amount of Al substitution is necessary to lower the hydrogen equilibrium pressure and this has presented difficulty in preparing alloys that are satisfactory in all aspects of discharge capacity, cycle life characteristics, high rate discharge characteristics, and low-temperature discharge characteristics.
In an attempt to make improvements on MmNiCoAl alloys having a comparatively small discharge capacity, MmNiCoMnAl alloys having part of Ni replaced by Mn have been recently proposed. The Mn substitution is effective for the purpose of increasing the discharge capacity of a battery but recently reported phenomenon in which Mn present in the neighborhood of the surface of alloy particles dissolves into the electrolyte upon repeated charge and discharge cycles has revealed the possibility that Mn substitution shortens the cycle life of the battery. Hence, if one effects Mn substitution, he also has to take a measure for preventing the deterioration of cycle life characteristics by inhibiting the dissolution of Mn into the electrolyte. However, no effective method for achieving this has yet been discovered.
Further, MmNiCoAl alloys in which Mm has high Ce content have had the problem of deterioration during storage since they contain easily oxidizable Ce. In the process of producing electrodes, the hydrogen absorbing alloy can sometimes be left in powder form for a prolonged time for several reasons such as the need for stockpiling for process levelling and the occurrence of problems in the production equipment. Care must be taken to insure that no changes will occur in the electrode characteristics of the alloy even in those cases, but alloys using Mm will experience a drop in initial discharge capacity, particularly when they are stored under hot and humid conditions for a prolonged time. This is due to the oxidation of the alloys. While there are several methods that can be used to avoid this problem, the most important thing is to prepare alloys that can be left to stand for a prolonged time without deterioration in their characteristics. However, in the case of Mm-using alloys which have a high content of Ce that is inherently prone to oxidation, the efforts to improve the storage characteristics of alloys have been limited even if they have good cycle life characteristics.
As described above, using Ce-rich and inexpensive Mm as a starting material for producing hydrogen absorbing alloys has involved various problems in association with each of the cases of Co-, Al- and Mn-substitution and it has been difficult to design alloys taking into account all factors of battery performance including discharge capacity, cycle life characteristics, high rate discharge characteristics, low-temperature discharge characteristics and long-term storage characteristics.